In recent years, an assembled circuit board has been made high in density, large in the number of layers and increasing in variety of using materials. Therefore, the necessity for an X-ray inspection capable of observing the interior or joint part of a component, in addition to conventional optical inspections has increased.
In particular, an X-ray inspection at high temperature capable of observing a change in crystalline structure or a change in the melted state by heating a sample to high temperatures can be used to observe a melted state and behavior on cooling of a solder which exists at a joint part of a component in real time. Therefore, this inspection is useful in analyzing a cause of defective generation at a solder joint part.
Where an X-ray inspection is performed on a sample under heating conditions, conventionally, there are often found cases that the sample heated outside an inspection system is set inside the inspection system and then subjected to observation. An X-ray inspection system having a heating apparatus is also proposed. However, at some of the heating elements necessary in the heating apparatus or at a part of the apparatus, a metal low in X-ray transmission is used. Since the metal absorbs, reflects, diffracts or scatters X-rays, a disadvantage is found that an X-ray receiving apparatus is unable to sufficiently receive an X-ray from an X-ray irradiation apparatus to result in a failure of conducting an accurate inspection. Thus, a heating apparatus which is capable of conducting a clear X-ray inspection under heating conditions has been required.
Further, in order to conduct an accurate inspection, it is necessary to heat a sample to high temperatures as rapidly and uniformly as in actual production. Therefore, a heating apparatus which is capable of performing rapid and uniform heating has been required. Still further, since components have been downsized, there is a growing demand for observing a very small component clearly at high magnification. In order to reduce a focal length, a heating apparatus has been required to be made thinner.
Thus, in Patent Document 1, an observation apparatus in which a heater containing a metal at some parts is separated from a sample piece to heat the sample with heated-air is proposed.
In Patent Document 2, a heating apparatus which uses ceramics formed into a plate shape as a material constituting the heating apparatus is also proposed.
In Patent Document 3, a reflow heating apparatus equipped with a heating unit which heats compressed air and feeds it is still also proposed.
However, according to the above-described conventional technologies, there has been a problem that, where a sample heated outside an inspection system is set inside the inspection system and subjected to observation, the sample decreases in temperature or unevenness in temperature distribution during the setting, thus resulting in a failure of controlling the temperature sufficiently. Further, there also has been a problem that a change in the state of the sample in association with a change in the temperature, for example, void occurrence on melting of solder or a change in a wet state, is unable to be observed accurately and in real time according to a required profile.
Further, in an X-ray inspection system equipped with heating apparatus, a metal low in X-ray transmission is used at some of the heating elements of the heating apparatus or at a part of the apparatus. And, there also has been a problem that the metal blocks the field of view for observing an X-ray image to result in a failure of making a sufficient observation.
In order to cope with these problems, the observation apparatus known by Patent Document 1 is provided with a high-temperature chamber which has a heater and a fan outside an observation path of a sample, thereby heating the sample by supplying heated-air from the high-temperature chamber.
However, in the above-described method, it has been difficult to elevate the temperature of a sample rapidly and also difficult to heat the sample in its entirety uniformly, thereby it has been difficult to control the temperature of the sample, resulting in a failure of making an observation in real time according to a required profile.
Further, the heating element is separated from the observation path, thereby an air sending passage and an air blowing mechanism of heated-air is required. Therefore, a problem is posed that a system is made larger to increase a focal length, by which it is impossible to observe a very small component clearly at high magnification. There also have been problems that an angle for observation is limited, a system is complicated and fabricated with difficulty and the system is more likely to malfunction.
In the heating apparatus known by Patent Document 2, ceramics formed into a plate shape are used as a material constituting a heating element, thereby avoiding blockage of an X-ray by the material constituting the heating apparatus. However, a problem is posed that a ceramics heater is slow in elevating temperatures, difficult in controlling temperatures, easily broken and low in durability. Further, since the heating apparatus is not provided with a mechanism for generating air flow within it, there is found a problem that unevenness in temperature distribution inside the apparatus causes.
At present, reflow soldering heating methods used for boards produced worldwide are mostly done by convection heating. This is because, in most cases, components are loaded above and below a glass epoxy board (the back surface is not flat) and therefore thermal conduction heating is not usable.
However, there is a significant difference in soldering behavior between thermal conduction heating and convection heating which is actually conducted on the market. This is because there is a difference in temperature elevation speed between various parts such as a board, a solder paste and a component and also there is a difference in the amount of the solder paste to be dried and oxidized.
For this reason, an inspection which heats an assembled circuit board by thermal conduction heating that may cause a phenomenon which does not take place in reality is meaningless in examining a mechanism of various types of soldering defects caused by convection heating. The above fact is known by technical experts of soldering as common knowledge.
Where a board is subjected to a motion image inspection with an X-ray, convection heating is needed for examining the soldering behavior which takes place in reality. However, conventional reflow heating furnaces for X-ray inspection which have been so far filed are mostly conducted by thermal conduction heating.
In addition, convection heating is described in some of the patent applications like that found in Patent Document 3. Since heated-air is supplied outside a field of view of an X-ray, the temperature distribution on a board is different from the actual temperature distribution and temperatures differ widely at various parts. Further, in the reflow heating apparatus known by Patent Document 3, heated compressed air is ejected to cause convection, thereby heating a sample. Therefore, a problem is posed that an air compressing mechanism is needed to result in a larger and complicated apparatus.